Because of the extended commercial use of a great number of liquid substances in both commercial and private enterprise, a large variety of liquid storage systems have been developed and placed throughout the earth. While many systems include storage tanks which are erected in above ground locations, frequently the maximum use of land space as well as safety and economic considerations result in the placement of liquid storage tanks and support structures such as connecting pipes, pumps, and the like below ground. In many instances, the entire liquid storage system is buried a substantial distance beneath the earth surface and access thereto is limited to the substantial effort and inconvenience of excavating the entire site to expose the system. One of the most commonly seen underground liquid storage facilities is found in great number throughout industrialized nations in the form of a gasoline filling station or service station. In such systems, one or more storage tanks are buried in the ground together with appropriate connecting pipes and pumping apparatus to supply motor fuel to one or more ground level dispensing devices. Most typically, each underground tank includes a vertical filler pipe extending to the surface for periodic replenishment of the stored fluid.
In an idealized circumstance, the underground liquid storage systems would be fabricated, sealed, placed on site and covered over with earth. Thereafter, in such an idealized situation, no further access to such underground facilities would be required. Unfortunately, however, buried liquid storage systems are subjected to substantial degradation from corrosion of materials, physical damage due to shifting earth structure surrounding and supporting the system and the general deterioration of sealing apparatus and "wear and tear". Thus such underground liquid storage facilities are virtually destined to leak eventually and provide an undesired leakage flow of their contained liquid into the surrounding soil.
While the loss of liquid to the surrounding soil due to leakage is of some economic concern to system owners and operators, the major concern caused by such leakage arises due to the great variety of potentially harmful and toxic substances typically stored in such underground systems. Because liquid leaking into the surrounding soil of a subsurface storage system eventually contaminates the surrounding soil and usually find its way to the nearby underground water supplies, the environmental impact of such leakage is extremely serious.
The environmental and safety concerns created in relation to leakage of such underground liquid storage systems has prompted various controlling agencies to require the periodic testing of such systems the difficulty arises in implementing such testing standards since the detection of such leakage is often an extremely difficult process. One of the most common methods used to detect leakage in underground systems involves the process of attaching a vertical standpipe or riser to the filler pipe orifice of the system and thereafter sealing the standpipe to the orifice. Next, the system is filled completely with liquid and the other discharge connections and pumps are purged of vapor and sealed. Thereafter, the standpipe is filled to a level sufficient to permit the measurement of its liquid level within the standpipe. The measurement then comprises observation and measurement of the changes in liquid level over time. Simply stated, a decrease in liquid level over time signals the existence of a leak within the system. While such methods would seem to be relatively simple and straight forward, they tend to be extremely inaccurate particularly in attempting to detect small leakage. Since the detection of small leakage requires an extended fluid level measurement time, other events and processes within the system detract from the measurement accuracy. For example, the storage tanks themselves over time are subjected to temperature variation and expand or contract changing the system volume. A similar effect occurs within the liquid itself which during temperature changes also expands or contracts providing a second source of erroneous volume change. As a result, the above-described method falls far short of providing the accuracy required to meet the various regulations and rules provided for leakage testing by various agencies.
There arises, therefore, a need in the art for a more accurate improved method of leakage testing for liquid storage systems which maintains its accuracy and reliability in the face of temperature changes and other detracting influences.